Blurring the lines between life and inanimate matter, biologists said they have created six alternatives to DNA and coaxed them to undergo evolution.
The work bolsters a prevailing hypothesis that life as we know it evolved from simpler life forms, no longer here, and that those evolved from something simpler. There may be no moment when the first life emerged, but instead an evolutionary process by which chemicals that most of us would consider nonlife gradually gave rise to living cells through natural selection.
The work on these code-carrying molecules, called XNAs, adds to a growing field of test-tube evolution, in which scientists are nudging code-carrying chemicals to evolve into drugs or other useful compounds.
Scientists don't have a universal definition for life, but they do agree that to qualify as life, an organism must be subject to natural selection. All life on Earth is related and uses the same basic building blocks, but life elsewhere might be put together differently, challenging scientists to recognize it.
"Everyone wants to know what aliens would use for DNA," said Steven Benner, a biochemist at the Foundation for Applied Molecular Evolution in Gainesville, Fla., who has synthesized artificial DNA but was not involved in the new study. "Lab experiments tell you about the possibilities in the universe."
Biochemist John Chaput, one of the authors of the XNA paper in the journal Science, said evolution can happen in a molecule as long as it can carry a code and copy itself. At that point you have something capable of heredity.
Such chemicals might be commonly thought of as primordial soup, but on a molecular scale they are highly organized. Like DNA, these XNA molecules string together four different chemical units into chains. In DNA, the four chemical units are identified by the letters A, T, C, and G. Those are attached to a backbone or scaffolding, which in DNA is made from sugars and chunks called phosphates. The XNAs use the same characters -- A, T, C, and G -- but the backbones that hold them together are different.
Scientists suspect that DNA evolved from a simpler cousin called RNA. DNA comes as a double strand of code twisted into a spiraling ladder, while RNA is a single strand.
Gerald Joyce, a researcher at the Scripps Research Institute in La Jolla, Calif., said one interesting property of the XNAs is they're more durable than DNA or RNA. XNAs don't biodegrade, since they are outside our biology and can't be eaten or broken up by enzymes associated with life as we know it.
That ruggedness might make XNAs useful for fishing out specific DNA sequences in genetic tests or other diagnostics, or for detecting contaminants in the environment. Chaput said they could be particularly important in a growing field of medicine in which scientists prompt chemicals to evolve into new drugs. It could also shed light on how life emerged on Earth, and on what living things might look like if they exist beyond our planet.
The Los Angeles Times contributed to this report.